The intimate sequence of events occurring during the formation of the drug-receptor complex are characterized by fundamental constants which include the dissociation constant (KA, KB, reciprocals of affinity for agonists and antagonists), and the thermodynamic parameters associated with the reaction, i.e., changes in Gibbs free energy reflected as changes in enthalpy and entropy. Agonists and antagonists may bind to receptors differently, with only agonists able to possibly induce a conformational change in the receptor; this change is assumed responsible for the information transfer which initiates events leading to a measureable effect. This possible agonist-induced change in conformation is reflected by the reaction thermodynamics, with agonist and antagonists having different characteristics. It is now accepted that opioids interact with at least three types of opioid receptor, the mu, delta and kappa. While the fundamental parameters of drug-receptor interaction have been studied for many classes of pharmacological agents (e.g. alpha & beta adrenergics, nicotinics, muscarinics, benzodiazepines) surprisingly few studies have been attempted with opioids. Selective opioid agonists and antagonists, together with a non-surmountable antagonist, with affinity for all receptors, have recently been developed; these novel tools now allow the determination of opioid constants. Additionally, opioid specific bioassays have been defined with only one type of receptor (i.e. rabbit vas deferens for the kappa receptor) or created through alkylation of non-relevant receptor leaving only one functional opioid receptor. This proposal suggests the determination of individual opioid receptor constants, and the thermodynamic parameters of opioid agonists and antagonists in multiple bioassays (guinea-pig ileum, mouse vas deferens, rabbit vas deferens, hamster vas deferens). The approach focuses on receptor constants at different temperatures. Such information allows calculation of thermodynamic parameters of agonist and antagonist binding. The specific questions which will be addressed include: (a) do opioid agonists and antagonists bind to the same type of opioid receptor in different tissues (i.e. are mu receptors in the GPI the same as mu receptors in the MVD); (b) are the three opioid receptors affected differently by temperature; (c) are there differences in opioid agonist and antagonist binding, and are peptide agonists and antagonists different from non-peptides; (d) how are the binding parameters affected by tolerance, or supersensitivity. Knowledge of these fundamental opioid parameters should be instrumental in the process of rational drug design, particularly for antagonists.